Method of carrying out endothermic gas reactions



Dec. 23, 1969 H. KUHNE 3,4 8

METHOD OF CARRYING OUT ENDOTHERMIC GAS'REACTIONS Original Filed May 19,1964 2 Sheets-Sheet 1 INVENTOR. HE INZ KUHNE BY M Fm (Mm ATTORNEYS Dec.23, 1969 KUHNE 3,485,888

METHOD OF CARRYING OUT ENDOTHERMIC GAS' REACTIONS Original Filed May 19,1964 2 Sheets-Sheet 2 INVENTOR. HE/NZ KUHNE BY A ATTORNEYS United StatesPatent O 3,485,888 METHOD OF CARRYING OUT ENDOTHERMIC GAS REACTIONSHeinz Kuhne, Kronberg, Taunus, Germany, assignor to Vickers-ZimmerAktiengesellschaft Planung und Bau von Industrieanlagen Originalapplication May 19, 1964, Ser. No. 368,540, now Patent No. 3,345,139,dated Oct. 3, 1967. Divided and this application Jan. 16, 1967, Ser. No.644,031 Claims priority, application Germany, May 24, 1963, Z 10 137 US.Cl. 260-679 2 Claims ABSTRACT OF THE DISCLOSURE Improved method ofcarrying out endothermic gas reac' tions by passing the gas or gases tobe reacted in an annular reaction space which is concentricallysurrounded by but separated from an annular heating chamber and throughwhich a heating gas flows in the same direction as the reactant gas.

This invention relates to improvements in a method of carrying outendothermic gas reactions. This application is a division of mycopending application Ser. No. 368,540, filed May 19, 1964, titled Jacketed Annular Reactor Unit, now Patent No. 3,345,139.

In carrying out endothermic gas reactions, one aims at raising thereactants to the requisite high reaction temperature at a speed greaterthan their rate of decay, supplying the heat necessary for the progressof the reaction, retaining the reaction mixture within the hightemperatures range for the space of time needed for the reaction to takeplace and then cooling it as quickly as possible to a temperature atwhich the reaction product is stable.

Gas reactions calling for conditions of this kind are, for example, theconversion of hydrocarbons containing from 1 to 4 carbon atoms intoacetylene, the conversion of methane and ammonia into hydrocyanic acidor the conversion of hydrocarbons containing from 2 to 6 carbon atomsinto ethylene and propylene. For thermodynamic and kinetic reasons,technically satisfactory yields can be obtained from such reactions onlyat temperatures above 1,300 C.1,500 C., the elevated temperature beingmaintained for a period of time of the order of 10* to 10 sec.,according to the reaction temperature.

The production of such high reaction temperatures for the very shorttimes referred to involves considerable difiiculty; and, in fact, allthe solutions to this problem that have been put forward hitherto stillbring certain disadvantages and weaknesses in their train.

High reaction temperatures and short sojourn times can be achieved, forinstance, with the aid of an electric arc, through which the reactantsare passed at high velocity. Another way is to carry out the reaction ina flame, a certain amount of the reactants being burnt to supply therequisite heat. With both these methods of producing the reaction,temperatures of upwards of 2,000 C. are achieved in the reactingmixture. Another method, which is basically similar to both of theforegoing, consists in injecting the reactants into a stream of highlyheated carrier gases.

The fundamental disadvantage of the solutions outlined above is that theintroduction of the heat of reaction and the temperature variations inthe reaction mixture cannot be controlled accurately enough to ensurerational utilisation of the energy put in and the adequate suppressionof side reactions. With the methods involving partial combustion orhighly heated carrier gases, moreover, the de- Patented Dec. 23, 1969sired end product can be obtained in pure form only by subsequentseparation processes, which are expensive.

Another suggestion that has been made is that a ceramic grid should beheated by the combustion of heating gas with air or oxygen, thereactants then being led through the heated space, the heat required forthe reaction being supplied by the grid. Apart from the fact that here,again, in such a regenerative process, the supply of the heat ofreaction and the temperature variations in the reaction mixture cannotbe controlled with sufiicient precision, it is also frequently foundthat the temperatures required for optimum reaction conditions are notachieved, owing to the temperature-sensitive nature of the controlequipment.

The disadvantages mentioned in connection with the processes describedabove can be eliminated by the use of reaction ovens, in which ceramictubes are led through a gas-heated chamber. The reaction gas flowswithin the tubes, so that any dilution or contamination of the gaseousproduct by the heating or combustion gases, consequent upon the indirectsupply of heat (by heat exchange), is avoided, while at the same timeadequate control of the heat of reaction is possible. With suchequipment of this kind as is known as technologists, temperatures of upto 1,500 C. can already be obtained.

To achieve short sojourn times (of the order of magnitude referred toabove) in such ovens, it is true, tubes of extremely small diameter arenecessary. This in turn means that the heat transmission surfaces arevery small, so that, for the transmission .of the large quantities ofheat required to supply the heat requirements of highly endothermicreactions, high wall temperatures are essential, and these can often notbe attained, for technical reasons bound up with the materials availableand the heating gases at ones disposal. With a view to overcoming thisdifficulty it has also been proposed that two tubes made of materialscapable of withstanding high temperatures should be placedconcentrically one within the other, to form an annular reaction chamberthrough which to pass the flow of reaction gas.

With reaction ovens of the type already described, however, only veryfew tubes can be accommodated in any one oven unit, since one has to besure that the same amount of heat will be supplied from all sides toevery tube, so that the wall temperature, resulting not only fromradiation of the oven walls and heating gases, but also by convectiveheat exchange with the heating gases, may always be uniform. If the heattransmission be uneven, then the uneven wall temperatures will cause thetubes to undergo deformation, and this, With the refractory materialsused, which are highly sensitive to mechanical stresses, may easily leadto breakage or, at the least the riskwith two tubes running one insidethe other-that these tubes may be shifted from their concentricposition, resulting in distortion of the normally very narrow annularreaction space. Again, there being very few reaction spacesand these ofsmall cross-sectional areathe performance of the oven naturally fallswell below the justifiable minimum for technical equipment.

The invention aims at overcoming this disadvantage of reaction ovensfitted up with annular reaction spaces, each formed by two concentriccylindrical walls of refractory material. This it achieves by virtue ofthe outer cylindrical wall of the annular reaction space beingconcentrically surrounded by an annular heating chamber through which aheating gas flows in the same direction as the reactant gas, while theinner cylindrical wall of the reaction space is formed by a cylindricalbody concentrically disposed within the outer cylindrical wall, as

3 disclosed and claimed in my aforesaid copending application Ser. No.368,540.

The solution thus proposed according to the invention enables the outercylindrical wall of each and every reaction space to be brought, bymeans of the heating gas, to a wall temperature that is well defined andwhich is not only uniform round the periphery, but also undergoesuniform variation throughout its length; in addition to which, byradiation from the outer cylindrical wall, a correspondingly uniformtemperature is also produced on the inner cylindrical wall of thereaction space; nor can there be any disturbing influence due toradiation from an adjacent reaction tube or from any outer wall of thereaction oven or due, either, to uneven radiation or convective heattransmission from the heating-gas space. This in turn enables amultiplicity of reaction spaces to be brought together within a singleoven unit and hence makes it possible to increase the performance ofeach oven unit several times over, while reducing the risk of breakagesas compared with the gas-heated reaction ovens in this type knownhitherto.

In one preferred form of the invention, a moulded body of refractorybrick is provided with a cylindrical opening, inside which a tube ofrefractory material is housed concentrically, and concentrically withinthis tube a cylindrical body is similarly placed. It is desirable for anumber of such cylindrical openings to be provided in one such mouldedbody and/ or for a number of moulded bodies to be put together to formone block reactor with the cylindrical openings lying parallel.

Some embodiments of the apparatus used in conjunction with the method ofthis invention will now be described by way of example, reference beingmade to the accompanying drawings, in which:

FIGURE 1 is a diagrammatic cross-section of one form of the jacketedannular reactor unit used in the method of this invention;

FIGURE 2 is a diagrammatic cross-section of another form of the jacketedannular reactor unit used in the method of this invention;

FIGURE 3 is a modification of the form shown in FIGURE 2; and

FIGURES 4a and 4b are further modifications of the form shown in FIGURE2.

The principle of the invention can best be explained by reference toFIGURE 1. In this example, an outer cylindrical tube 1, an intermediatetube 2 and an inner cylindrical insertion 3 are arranged concentrically.The cylindrical insertion 3 may be solid as shown, but for reasons ofmanufacturing technique, as well as on account of weight considerations,it may with advantage consist of a tube closed at one end, which willserve the same purpose. The annular space 4 formed between the innerwall 11' of the outer tube 1 and outer wall 2a of the intermediate tube2, is intended for the flow of heating gases, while the annular space 5formed between the inner wall 21 of the intermediate tube 2 and theouter wall 3a of the insertion 3, is intended for the flow of reactiongas, the heating and reaction gases flowing in the same direction.

The result of this form of construction is that the heating gases supplyheat to the inner face 11' of the tube 1 and to the outer face 2a of thetube 2 by convective transfer, the magnitude of which can bepre-determined by the gas velocity. By reason of the uniformcross-sectional area of the annular space 4 and of the uniform speed offlow of the heating gas, this transference of heat is completely uniformround the periphery of the tube and subject to uniform variation alongthe length of the tube, according to the temperature gradient of theheating gas in the direction of flow. Similar conditions also apply tothe heat transference by radiation from the inner wall It of the tube 1to the outer wall 2a of the tube 2, which, as a secondary heatingeffect, brings about a definite further rise in the heat output passingby convection from the heating gas to the tube 2. The result is thatthere is a precisely defined transmission of heat in each surfacecomponent of tube 2 and hence a precisely defined wall temperature.

The foregoing considerations concerning the heat output side (tubes 1and 2) apply similarly to the heat intake side constituted by the tube 2and the cylindrical insertion 3. The inner wall 2i of the tube 2 (heatedfrom outside) yields up heat by convective transfer to the reaction gasflowing in the annular space 5. At this same time, radiant heat passesfrom the wall surface 2i to the outer face 3a of the cylindricalinsertion 3, so that this face acts as a secondary heating surface. Onthe heat intake side, too, by reason of the concentric arrangement ofthe faces 21' and 3a and the uniform speed of flow of the reaction gasin the annular space 5, the temperature conditions are uniform round theperiphery and subject to uniform variation in the direction of flow.

The two tubes 1 and 2, as well as the cylindrical insertion 3, are of arefractory material, ordinarily a sintered ceramic, which is extremelysensitive to differences in wall temperature. The complete and entirelyreliable uniformity of the wall temperatures obtained with thearrangement just described now enables a large number of such tubes tobe assembled together, without adjacent tubes and/or the outer walls ofthe oven exercising any secondary influence counter to the unformity ofwall temperatures achieved. The performance of individual oven units cantherefore be materially improved, while simultaneously the operationalreliability is enhanced and, as already mentioned, the specific outputof the heating surface areas is increased.

At the same time, the fact that the reaction and heating gases flow inthe same direction avoids the production of excessively high localmaximum temperatures (each as would arise, particularly at the point ofentry of the heating gas into the annular space 4, if the gases flowedin opposite directions) and thus has the effect of equalising thetemperature conditions along the length of the oven. This also meansthat the heating gas can be admitted at higher temperatures, the air ofcombustion being recuperatively pre-heated by the burnt gases flowingfrom the reaction oven. The adoption of this measure does not lead toany increase in the specific transference of heat at the entry to theoven and so to a shortening of the duration of heating or a reduction inthe length of the reaction gas heating zone, but it results in aconsiderable improvement in the thermal efficiency of the reaction thattakes place.

Since the available refractory materials can often not be keptsufiiciently gastight, particularly at high operating temperatures, thereactor may with advantage be run in such a way that the reaction gasand the heating gas, flowing in the same direction, are at the samepressure and exhibit the same pressure gradient throughout the entirelength of the tubes. Since the How resistance of a gas is dependent onvolume, this can be effected by means of suitable volume regulatingdevices, which in themselves are well known and hence need not befurther described in detail as lying Within the scope of the presentinvention. If, however, some slight pressure difference between thereaction and heating gases can be tolerated, the reaction gas should beat a somewhat higher pressure than that of the heating gas, to avoidcontamination of the reaction product.

FIGURE 2 shows one advantageous form which the invention may take. Inthis, in contradistinction to the form shown in FIGURE 1, the outer tube1 is replaced by a moulded brick 6-of square cross-section, forexamplewhich is formed with a circular-section passage 7. This passage 7fulfills the function of the inner wall 11 of the tube 1, describedabove. As with the form of construction illustrated in FIGURE 1, theintermediate tube 2 and the cylindrical insertion 3 are arrangedconcentrically within the pasasge 7, so that annular spaces 4 and 5 arecreated for the heating gas and the reaction gas, respectively.

The moulded'brick 6, as are the other components of the equipment, isformed of a refractory material. It may be made, for instance, by thecompression of a refractory material in powder form (such as corundium,zirconia or the like) in a mould, followed by sintering. The tube 2 andthe insertion 3 (which can both be made in the usual manner) should, butneed not, consist of the same refractory material as the moulded brick6.

As will be clear from FIGURE 2, several such moulded bricks 6 can beassembled to form a reactor block, with passages 7 (and hence alsoannular reaction spaces 5) lying parallel to one another. As each of thereaction spaces 5 is surrounded by an outer heating space 4, completegas impermeability is not absolutely essential in the moulded bricks 6.Whatever traces of heating gas may escape outwards from one of theannular spaces 4 will at most reach the corresponding heating-gas spacein an adjacent moulded brick. This will not cause any trouble, so longas the leakage is not excessive. With a reaction oven built up frommoulded bricks, moreover, the normally rather poor heat conductivity ofsuch refractory materials as can be employed in the present case (whichapplies to zirconia, for instance) has no further harmful effect, sincethe annular space for the heating gas surrounds the annular space forthe reaction gas.

It is not essential for the moulded bricksto be square in section.FIGURE 3 shows an alternative method of construction, in which mouldedbricks 8 are hexagonal in section and are assembled with the tube unitsin staggered formation.

A further possibility lies, as shown in FIGURES 4a and 4b, in the use ofslabs shown at 9 and 10 respectively, formed with a plurality ofparallel passages 7 which, as shown in FIGURE 4a may be disposed in rowsand columns or, as shown in FIGURE 4b may be staggered.

In one practical example-particularly suitable for the pyrolyticconversion of methane into acetylene-of the individual forms of theinvention already described, the Width of the reaction-gas space 5 maybe approximately 4 mm. and that of heating-gas space 4 approximately 8mm., for an effective tube length of 1.9 rn. If the insertion 3, in theform of a tube closed at one end and having a wall thickness of about1.5 mm., has an outside diameter of approximately 7 mm., and if the wallthickness of the inner tube 2 is approximately 2.5 mm., the insidediameter of outer tube 1 (or of the passage 7 in the moulded brick, asthe case may be) will be about 36 mm. If such a reactor be run with anadmission temperature of 2,200 C. for the heating gas and an admissiontemperature of 1,000 C. for the reaction gas, both gases being admittedat the same pressure of a trifle over 1 atm. abs. and having the samelow pressure gradient along the reactive length, then, if the speed offlow of the reaction gas in annular space 5 be of the order of 100m./sec., the heating zone will be approximately 0.5 m. long and thereaction zone approximately 1.4 m. long, the mean time of sojourn of thereaction gas in the reaction zone being approximately 10 sec. and itsmaximum temperature about 1,600 C. to 1,650 C., and the reaction gaswill leave the annular reaction space (to pass into a subsequent coolingstage) at about 1,500 C., the heating gas leaving the annular heatingspace at about 1,750 C. This mode of operation means that, for eachtube, about 6,300 Kcal./hr. of perceptible heat and heat of reactionwill be transformed and that the specific heat output per unit area,referred to the mean heating-tube diameter (17.5 mm.), lies in theregion of 60,000 Kcal./ sq. m./hr. It has also been found that, inpractice, any number of reactors desired may be brought together to forma single oven unit, without any difiiculties arising ornotwithstandingthe length of the tubes-even any intolerable deviations being observedfrom the nominal dimensions of spaces 4 and 5.

What I claim is:

1. In an endothermic gas reaction process which includes the steps of(1) passing a reactant gas into a recation zone, (2) heating saidreactant gas to reaction temperature at a speed greater than the rate ofdecay until a reaction product is formed, and (3) rapidly coolingreaction product gas to a temperature at which said product is stable,the improvement which comprises:

(a) passing said reactant gas through a first annular reaction zone,

(b) passing a heating gas through a separate, second annular zoneconcentrically and outwardly spaced from said annular reaction zone,said reactant gas being heated substantially only by transfer of heatfrom said heating gas to said reactant gas,

(0) maintaining both said reactant gas and said heating gas flowingthrough their respective annular Zones in the same direction,

(d) maintaining the pressure of said reactant gas not less than thepressure of said heating gas to prevent any substantial amount ofcontamination of reactant gas or reaction product with heating gas,

(e) regulating the flow of said reactant gas and heating gas to maintaina substantially similar pressure gradient throughout the length of bothsaid annular zones,

(f) maintaining said reactant gas in said annular reaction zone for aresidence time on the order of from about 10* to 10- seconds andsufficient to complete reaction,

(g) withdrawing reaction product from said first annular zone withoutneed for separation of heating gas therefrom, whereby thermal efliciencyof the reaction is improved, variations in temperature in said reactionzone are controlled pursuant to a predetermined gradient withoutformation of hot spots causing product decay, and side reactions aresuppressed.

2. A process as in claim 1 wherein:

(a) said reaction gas is methane,

(b) the pressure of said reactant gas admitted into said annularreaction zone and said heating gas admitted into said annular heatingzone being about 1 atmosphere absolute,

(c) the flow of said reaction gas being about m./sec., and

(d) the reactant gas being heated to a maximum temperature between about1,6001,650 C., whereby said methane is pyrolitically reacted in saidannular reaction zone to produce acetylene.

References Cited UNITED STATES PATENTS 1,934,610 11/1933 Wheeler 23-1512,069,545 2/1937 Carlisle et a1. 23-151 2,596,421 5/1952 McKinnis 23-1512,779,662 1/1957 Frey 23-202 2,823,982 2/ 1958 Saladin et al. 23-2022,937,923 5/1960 Shapleigh 23-1 2,942,043 6/1960 Rummert 23-1 X R3,004,822 10/ 1961 Poorman et a1. 23-1 3,345,139 10/1967 Kuhne 23-1 XREDWARD STERN, Primary Examiner U.S.Cl.X.R.

